Magnesium alloy sheet and magnesium alloy structural member

ABSTRACT

Provided are a magnesium alloy sheet having excellent formability in plastic forming, such as press forming, and a magnesium alloy structural member. The magnesium alloy sheet is obtained by subjecting a magnesium alloy to rolling and has a cross section parallel to the thickness direction of the magnesium alloy sheet, in which, when the length of the major axis and the length of the minor axis of each of crystal grains in the cross section are determined, an aspect ratio is defined as the ratio of the length of the major axis to the length of the minor axis (length of major axis/length of minor axis), and crystal grains having an aspect ratio of 3.85 or more are defined as elongated grains, the area fraction of the elongated grains in the cross section is 3% to 20%.

TECHNICAL FIELD

The present invention relates to a magnesium alloy sheet obtained by rolling, and a magnesium alloy structural member made from the magnesium alloy sheet. More particularly, the invention relates to a magnesium alloy sheet having excellent plastic formability.

BACKGROUND ART

Magnesium alloys, which are lightweight and have excellent specific strength and specific rigidity, have been used as materials constituting various structural members, such as housings of mobile electric/electronic devices, e.g., cellular phones and laptop computers, and parts of automobiles.

Magnesium alloys typically have a hexagonal close-packed crystalline structure, in which slip planes at low temperatures, such as at room temperature, are basal planes only. Therefore, existing magnesium alloy structural members are typically cast materials produced by a die casting process or thixomolding process.

In recent years, as described in Patent Literatures 1 and 2, a method has been studied in which a magnesium alloy is subjected to rolling, and the resulting rolled sheet is subjected to plastic forming, such as press forming. When rolling and plastic forming, such as press forming, are performed on a magnesium alloy, plastic formability can be enhanced by performing warm working in which the material is heated and a processing jig, such as reduction rolls or a press mold, is heated as described in Patent Literature 1.

CITATION LIST Patent Literature

PTL 1: Japanese Unexamined Patent Application Publication No. 2011-131274

PTL 2: Japanese Unexamined Patent Application Publication No. 2005-298885

SUMMARY OF INVENTION Technical Problem

In producing a plastic-formed structural member, such as a press-formed body, composed of a magnesium alloy, there is a demand for development of a magnesium alloy sheet having excellent plastic formability. Furthermore, there is a demand for development of a magnesium alloy sheet which enables production of a plastic-formed structural member with excellent mechanical properties, such as strength and impact resistance.

Patent Literature 1 discloses that by performing warm rolling while controlling the temperatures of the material and reduction rolls to specific values, it is possible to obtain a rolled sheet having excellent plastic formability. In this rolled sheet, a sufficient amount of processing strain is introduced by rolling and coarsening of crystal grains is suppressed by the temperature control described above. As a result, dynamic recrystallization is caused during press forming, and excellent plastic formability is exhibited. In general, mechanical properties of a magnesium alloy depend on the size of crystal grains. As crystal grains become finer, strength and elongation improve. In the rolled sheet described above, coarsening of crystal grains is suppressed, i.e., crystal grains are fine. Therefore, the rolled sheet has excellent strength and elongation, and the press-formed body obtained using the rolled sheet as a material also has excellent strength and impact resistance.

However, when a magnesium alloy having a hexagonal close-packed structure is subjected to rolling, the c-axis of the crystal (the axis perpendicular to the (0001) plane which is the basal plane) is oriented perpendicular to the rolled surface (surface of the material formed by being brought into contact with a reduction roll among surfaces of the material). That is, the rolled sheet has a structure in which the (0001) plane is oriented parallel to the rolled surface. Therefore, the rolled sheet has anisotropy in plastic forming, is hard to bend in a given direction, and has poor plastic formability. Consequently, there is a demand for development of a magnesium alloy sheet in which anisotropy in plastic forming is reduced.

Patent Literature 2 discloses a method for producing a magnesium alloy sheet in which after warm rolling, both treatment with a roll leveler and recrystallization heat treatment are repeated successively a plurality of times. In the rolled sheet obtained by this production method, the c-axis (the {0002} plane) is inclined with respect to the rolled surface, and therefore, bend forming or the like can be performed even at low temperatures. On the other hand, the rolled sheet has poor mechanical properties (in particular, strength and rigidity) and is easily deformed even at room temperature, and dent deformation can be caused by impact, such as dropping.

In addition, a magnesium alloy incorporated with about 10.5% to 16% by mass of Li has a cubic crystalline structure, and therefore, it can be subjected to press forming even at room temperature. However, this magnesium alloy is easily deformed at room temperature and has poor strength and impact resistance. Furthermore, since this magnesium alloy contains a large amount of Li, it has poor corrosion resistance.

Accordingly, it is an object of the present invention to provide a magnesium alloy sheet which can constitute a magnesium alloy structural member having excellent strength and impact resistance and which has excellent plastic formability. It is another object of the present invention to provide a magnesium alloy structural member having excellent strength and impact resistance.

Solution to Problem

A magnesium alloy sheet according to the present invention is obtained by subjecting a magnesium alloy to rolling and has a cross section parallel to the thickness direction of the magnesium alloy sheet, in which, when the length of the major axis and the length of the minor axis of each of crystal grains in the cross section are determined, an aspect ratio is defined as the ratio of the length of the major axis to the length of the minor axis, and crystal grains having an aspect ratio of 3.85 or more are defined as elongated grains, the area fraction of the elongated grains in the cross section is 3% to 20%.

Advantageous Effects of Invention

The magnesium alloy sheet according to the present invention has excellent plastic formability.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1(A) is an inverse pole figure orientation map (IPF Map) of elongated grains by an SEM-EBSD technique in Sample No. 2, FIG. 1(B) is a graph showing the relationship between the aspect ratio and the occurrence frequency of crystal grains in Sample No. 2, and FIG. 1(C) is a pole figure of (0001) planes of elongated grains in Sample No. 2.

FIG. 2(A) is an angle graph in the rolling direction (RD direction) of crystal grains with the angle in the sheet width direction from the normal direction being within 5° regarding (0001) planes of elongated grains in Sample No. 2, and FIG. 2(B) is an angle graph in the sheet width direction (TD direction) of crystal grains with the angle in the rolling direction from the normal direction being within 20° regarding (0001) planes of elongated grains in Sample No. 2.

FIG. 3(A) is a view used for explaining selection of elongated grains, and FIG. 3(B) is a view used for explaining selection of crystal grains with a specific angle from the pole figure of (0001) planes.

FIG. 4(A) is an inverse pole figure orientation map (IPF Map) of elongated grains by an SEM-EBSD technique in Sample No. 3, FIG. 4(B) is a graph showing the relationship between the aspect ratio and the occurrence frequency of crystal grains in Sample No. 3, and FIG. 4(C) is a pole figure of (0001) planes of elongated grains in Sample No. 3.

FIG. 5(A) is an angle graph in the rolling direction (RD direction) of crystal grains with the angle in the sheet width direction from the normal direction being within 5° regarding (0001) planes of elongated grains in Sample No. 3, and FIG. 5(B) is an angle graph in the sheet width direction (TD direction) of crystal grains with the angle in the rolling direction from the normal direction being within 20° regarding (0001) planes of elongated grains in Sample No. 3.

FIG. 6(A) is an inverse pole figure orientation map (IPF Map) of elongated grains by an SEM-EBSD technique in Sample No. 4, FIG. 6(B) is a graph showing the relationship between the aspect ratio and the occurrence frequency of crystal grains in Sample No. 4, and FIG. 6(C) is a pole figure of (0001) planes of elongated grains in Sample No. 4.

FIG. 7(A) is an angle graph in the rolling direction (RD direction) of crystal grains with the angle in the sheet width direction from the normal direction being within 5° regarding (0001) planes of elongated grains in Sample No. 4, and FIG. 7(B) is an angle graph in the sheet width direction (TD direction) of crystal grains with the angle in the rolling direction from the normal direction being within 20° regarding (0001) planes of elongated grains in Sample No. 4.

DESCRIPTION OF EMBODIMENTS Description of Embodiments of Present Invention

The present inventors produced rolled sheets composed of a magnesium alloy under various conditions, and the rolled sheets, as material sheets, were subjected to press forming to examine formability. As a result, it has been found that a material sheet in which breakage, surface roughening, and the like are unlikely to occur even when subjected to high deformation and from which a formed body having excellent surface texture is obtained has a specific structure. Furthermore, it has been found that the resulting formed body has excellent strength and impact resistance. Moreover, it has been found that the material sheet can be produced by using a material for rolling having a specific structure and by subjecting the material for rolling to warm rolling under specific conditions. The present invention has been achieved on the basis of the findings described above. First, embodiments of the present invention will be enumerated and described.

(1) A magnesium alloy sheet according to an embodiment of the present invention is obtained by subjecting a magnesium alloy to rolling and has a cross section parallel to the thickness direction of the magnesium alloy sheet, in which the area fraction of elongated grains in the cross section is 3% to 20%. The elongated grains are defined as crystal grains having an aspect ratio of 3.85 or more in the cross section when the length of the major axis and the length of the minor axis of each of crystal grains in the cross section are determined, and the aspect ratio is defined as the ratio of the length of the major axis to the length of the minor axis (length of the major axis/length of the minor axis).

The structure in which the long crystal grains with a specific size (elongated grains) are present in the specific range can be considered as a structure in which the orientation is disturbed to a certain extent. In the magnesium alloy sheet according to the embodiment having such a specific structure, anisotropy in plastic forming can be reduced and excellent formability is exhibited compared with the structure in which all crystal grains are oriented in a certain direction. Furthermore, in the magnesium alloy sheet according to the embodiment, crystal grains other than the elongated grains are fine because they have been subjected to rolling and constitute a structure having certain orientation (structure in which the c-axis is oriented perpendicular to the rolled surface). Therefore, in the magnesium alloy sheet according to the embodiment, it is possible to suppress a decrease in strength due to the presence of the elongated grains, and the fine, oriented structure allows high strength and elongation. Thus, the magnesium alloy sheet has excellent strength, elongation, and impact resistance. Note that a typical example of the cross section in which the area fraction of elongated grains is 3% to 20% is a cross section parallel to the rolling direction.

(2) In an example of the magnesium alloy sheet according to the embodiment, when a pole figure of (0001) planes of the elongated grains is taken, crystal grains with the angle θ_(TD) in the sheet width direction in the (0001) planes of the elongated grains within 5° are selected, and the angle θ_(RD) in the rolling direction in the (0001) planes of the selected crystal grains is checked, the peak of the angle θ_(RD) in the rolling direction is present at 9° or more from the normal direction.

To put it plainly, in the example described above, there are many crystal grains with the (0001) plane inclined to the rolling direction (hereinafter, referred to as RD-inclined elongated grains). That is, the example has a structure which includes crystal grains oriented in different directions (RD-inclined elongated grains) and which is random to some extent. Therefore, anisotropy in plastic forming can be sufficiently reduced and excellent plastic formability is exhibited compared with the structure which is substantially composed of only crystal grains with the c-axis oriented perpendicular to the rolled surface.

(3) In an example of the magnesium alloy sheet according to the embodiment, when a pole figure of (0001) planes of the elongated grains is taken, crystal grains with the angle θ_(RD) in the rolling direction in the (0001) planes of the elongated grains within 20° are selected, and the angle θ_(TD) in the sheet width direction in the (0001) planes of the selected crystal grains is checked, the total area fraction of crystal grains with the angle θ_(TD) in the sheet width direction from the normal direction being −20° or less and crystal grains with the angle θ_(TD) in the sheet width direction from the normal direction being +20° or more is 20% to 70% relative to all the elongated grains.

To put it plainly, the example described above includes crystal grains with the (0001) plane largely inclined to the sheet width direction (hereinafter, referred to as TD-inclined elongated grains) in a specific range. That is, the example has a structure which includes crystal grains oriented in different directions (TD-inclined elongated grains) and which is random to some extent. Therefore, anisotropy in plastic forming can be sufficiently reduced and excellent plastic formability is exhibited compared with the structure which is substantially composed of only crystal grains with the c-axis oriented perpendicular to the rolled surface. Moreover, in the example, since the content of TD-inclined elongated grains is in a specific range, degradation in mechanical properties due to the presence of TD-inclined elongated grains is suppressed, and excellent strength and impact resistance are exhibited.

(4) In an example of the magnesium alloy sheet according to the embodiment, the average cross-sectional area of the elongated grains is 600 μm² or less.

In the example described above, the elongated grains are small and unlikely to act as starting points for breakage during plastic forming, and therefore, excellent plastic formability is exhibited.

(5) In an example of the magnesium alloy sheet according to the embodiment, the magnesium alloy contains 8.3% to 9.5% by mass of aluminum (Al).

The magnesium alloy containing Al in the specific range described above (hereinafter, referred to as the high Al-content magnesium alloy) has excellent mechanical properties (in particular, strength) and corrosion resistance. Accordingly, the embodiment has excellent plastic formability because of the specific structure including the elongated grains and also has excellent mechanical properties (in particular, strength) and corrosion resistance because of the specific composition.

(6) In an example of the magnesium alloy sheet according to the embodiment, the cross-sectional area of each of the elongated grains is more than 25 μm² and 5,000 μm² or less.

In the example, since each of the elongated grains is small and unlikely to act as a starting point for breakage, excellent plastic formability is exhibited.

(7) A magnesium alloy structural member according to an embodiment of the present invention is obtained by subjecting at least part of the magnesium alloy sheet according to the embodiment to press forming.

The magnesium alloy structural member according to the embodiment is produced using, as a material, the magnesium alloy sheet according to the embodiment having excellent plastic formability, and therefore has high productivity and high shape accuracy and dimensional accuracy. Furthermore, the magnesium alloy structural member according to the embodiment is composed of the magnesium alloy sheet having excellent mechanical properties, such as strength and elongation, and therefore has excellent mechanical properties, such as strength, rigidity, and impact resistance.

Details of Embodiments of Present Invention

The magnesium alloy sheet according to the embodiment and the magnesium alloy structural member according to the embodiment will be described in detail below.

[Magnesium Alloy Sheet]

(Composition)

The magnesium alloy sheet according to the embodiment and the magnesium alloy structural member according to the embodiment are each composed of any of magnesium alloys having various compositions in which various additive elements are added to Mg (balance: Mg and impurities, Mg: 50% by mass or more).

Examples of the additive element include at least one element selected from the group consisting of Al, Zn, Mn, Si, Be, Ca, Sr, Y, Cu, Ag, Sn, Li, Zr, Ce, Ni, Au, and rare-earth elements (excluding Y and Ce). In particular, a Mg—Al-based alloy containing Al has excellent strength, rigidity, impact resistance, and the like and also has excellent corrosion resistance. The Al content is, for example, 0.1% by mass or more. As the Al content increases, strength and corrosion resistance tend to become higher. However, when the Al content exceeds 12% by mass, plastic formability is degraded. Therefore, the Al content is preferably 12% by mass or less, and more preferably 11% by mass or less.

The content of the elements other than Al is, for example, 0.01% to 10% by mass, or 0.1% to 5% by mass. In particular, a magnesium alloy containing 0.001% by mass or more in total, preferably 0.1% to 5% by mass in total of at least one element selected from the group consisting of Si, Sn, Y, Ce, Ca, and rare-earth elements (excluding Y and Ce) has excellent heat resistance and flame retardance. Examples of the impurities in the magnesium alloy include Fe.

More specifically, examples of the Mg—Al-based alloy include AZ-based alloys (Mg—Al—Zn-based alloys, Zn: 0.2% to 1.5% by mass), AM-based alloys (Mg—Al—Mn-based alloys, Mn: 0.15% to 0.5% by mass), AS-based alloys (Mg—Al—Si-based alloys, Si: 0.2% to 6.0% by mass), AX-based alloys (Mg—Al—Ca-based alloys, Ca: 0.2% to 6.0% by mass), and AJ-based alloys (Mg—Al—Sr-based alloys, Sr: 0.2% to 7.0% by mass) specified in the standards of American Society for Testing and Materials (ASTM). Other examples include Mg—Al-RE-based alloys (RE: rare-earth element, RE: 0.001% to 5% by mass, preferably 0.1% by mass or more).

Among Mg—Al-based alloys, alloys containing more than 7.2% by mass of Al, in particular, alloys containing 8.3% to 9.5% by mass of Al have more excellent mechanical properties, such as strength and impact resistance and corrosion resistance, which is preferable. Specific examples of the composition include AZ91 alloys and AZX911 alloys containing, in addition to Al, 0.5% to 1.5% by mass of Zn.

(Shape)

The magnesium alloy sheet according to the embodiment is typically a rectangular sheet having a rectangular planar shape. By appropriately cutting or punching, a sheet having a desired planar shape, such as a circular, elliptic, or polygonal shape, can be obtained. Furthermore, it is also possible to obtain a coil by spirally winding a long rectangular sheet.

(Thickness, Width, and Length)

The magnesium alloy sheet according to the embodiment typically has a uniform thickness overall. The thickness can be selected appropriately. When the magnesium alloy sheet is used as a material for a plastic-formed structural member, the plastic-formed structural member has a thickness substantially equal to the thickness of the material sheet. Therefore, as the thickness of the magnesium alloy sheet is decreased, it is possible to achieve reduction in thickness, size, and weight of the plastic-formed structural member. Specifically, for example, the thickness is 0.1 mm or more and 2.5 mm or less, or 2 mm or less, in particular, 1.5 mm or less. Particularly, a thickness of 0.3 to 1.2 mm is easy to use. Furthermore, the magnesium alloy sheet may have portions that have partially different thicknesses, such as through-holes, recesses, and protrusions.

The width and length (maximum distance between two points on the outline in the case of an irregular-shaped sheet, such as a circular sheet, elliptical sheet, or polygonal sheet) of the magnesium alloy sheet can be appropriately selected. For example, in the case of a rectangular sheet, when the rectangular sheet is a wide sheet with a width of 100 mm or more, or 200 mm or more, in particular, 250 mm or more, and is used as a material for a plastic-formed structural member, it is possible to produce plastic-formed structural members having various sizes from small-sized structural members, such as parts of mobile devices, to large-sized structural members, such as parts of transportation apparatuses. Furthermore, for example, in the case of a rectangular sheet, when the rectangular sheet is a long sheet with a length of 50 m or more, 100 m or more, 200 m or more, or 400 m or more, and is used as a material for a plastic-formed structural member, the material can be continuously supplied to a plastic forming device, and the plastic-formed structural member can be mass-produced. When a coil is formed by spirally winding such a long sheet, transport and supply to a plastic forming device are facilitated.

(Form)

The magnesium alloy sheet according to the embodiment has at least been subjected to rolling. Specific examples thereof include a rolled sheet (as rolled) and a treated sheet which has been subjected to treatment described below after rolling. Examples of the treatment include heat treatment (annealing) for removing strain introduced during rolling, polishing, straightening, anticorrosion treatment, such as chemical conversion treatment or anodic oxidation treatment, coating, hairline finish, and decorative treatment, such as diamond cutting and etching. By carrying out any of these treatments in a temperature range lower than the recrystallization temperature of the alloy constituting the magnesium alloy sheet, the treated sheet substantially maintains the structure immediately after rolling (specific structure including elongated grains).

(Mechanical Properties)

The magnesium alloy sheet according to the embodiment has the specific structure, which will be described later, and has been subjected to rolling, and therefore has excellent mechanical properties compared with the cast sheet composed of a magnesium alloy having the same composition. Although depending on the composition, for example, in the case where a magnesium alloy sheet is composed of a high Al-content magnesium alloy, such as an AZ91 alloy, it is possible to obtain a magnesium alloy sheet having a tensile strength of 270 to 450 MPa and a 0.2% proof stress of 220 to 350 MPa, and a magnesium alloy sheet having an elongation at break of 1% to 15% (at room temperature for each). Note that being composed of a high Al-content magnesium alloy and having a tensile strength and a 0.2% proof stress satisfying the ranges described above support that the sheet has been subjected to rolling.

(Structure)

The magnesium alloy sheet according to the embodiment basically has a hexagonal close-packed crystalline structure and at least one long crystal grain, which is referred to as the elongated grain, is present therein. The magnesium alloy sheet has a structure containing the elongated grains in a specific range (specific area fraction).

The elongated grains are defined as crystal grains having an aspect ratio of 3.85 or more when a cross section is taken parallel to the thickness direction of the magnesium alloy sheet, the length of the major axis and the length of the minor axis of each of crystal grains in the cross section are determined, and the aspect ratio is defined as the ratio, length of the major axis/length of the minor axis. Detailed description will be made later on how to take the cross section, a method for measuring the length of the major axis and the length of the minor axis, and a method for selecting elongated grains. The present inventors have studied and found that the rolled sheet subjected to rolling under the specific conditions described below has very many crystal grains having an aspect ratio of about 1.4 to 3.4 and some crystal grains having a high aspect ratio (20% or less in terms of area fraction). Since it is believed that crystal grains that affect plastic formability are long to some extent, in the magnesium alloy sheet according to the embodiment, crystal grains having an aspect ratio of 3.85 or more are defined as elongated grains. If crystal grains other than elongated grains are small to some extend (preferably about 10 μm or less in terms of average crystal grain size), the elongated grains may have a high aspect ratio of, for example, 10 or more.

Elongated grains tend to be stretched in the rolling direction (travelling direction of the material) as rolling proceeds. Therefore, in order to properly select elongated grains, it is considered to be appropriate to take a cross section of the magnesium alloy sheet cut along a plane parallel to both the thickness direction and the rolling direction (so-called longitudinal section) and to measure the length of the minor axis and the length of the major axis of each of crystal grains in the cross section. In the case where it is possible to determine the rolling direction of the magnesium alloy sheet, a cross section parallel to both the thickness direction and the rolling direction (i.e., longitudinal section) may be defined as the measurement cross section. In the case where the magnesium alloy sheet is wound, for example, in a coil shape, since the longitudinal direction usually corresponds to the rolling direction, a cross section parallel to the longitudinal direction may be defined as the measurement cross section. In the case where the magnesium alloy sheet is a rectangular sheet, a circular sheet, or the like and it is not possible to determine the rolling direction, any cross section parallel to the thickness direction is defined as the measurement cross section, and presence or absence of a cross section in which the area fraction of elongated grains having an aspect ratio of 3.85 or more is 3% to 20% (which is, hereinafter, referred to as the relevant cross section) is determined. In the case where there is a relevant cross section, a direction parallel to the relevant cross section is defined as the rolling direction of the magnesium alloy sheet and a direction perpendicular to both the rolling direction and the thickness direction is defined as the sheet width direction.

The area fraction of elongated grains is obtained by summing up the area of at least one elongated grain present in a given field of view in the cross section and calculating the ratio of the total area of elongated grains to the area of the field of view. When the area fraction of elongated grains is 3% or more, anisotropy in plastic forming is reduced by the elongated grains, plastic formability can be enhanced, and for example, the limiting drawing ratio can be increased. As the area fraction of elongated grains increases, plastic formability tends to become better, and it is possible to achieve an increase in the limiting drawing ratio and suppression of occurrence of cracks. However, when there are too many elongated grains, the elongated grains themselves may act as starting points for breakage to cause cracks, and surface roughening due to irregularities of the elongated grains may occur, resulting in deterioration in the surface texture or reduction in productivity. Therefore, the area fraction of elongated gains is set at 20% or less. More preferably, the area fraction of elongated gains is 5% to 15%.

When the elongated grains are excessively large, as described above, breakage and surface roughening are likely to be caused. Therefore, the average cross-sectional area of the elongated grains is preferably 600 μm² or less. When the elongated grains are excessively small, it becomes difficult to obtain the effect of reducing anisotropy in plastic forming. Therefore, it is believed to be preferable that the average cross-sectional area of elongated grains be about 100 μm² or more. Furthermore, the cross-sectional area of each of the elongated grain is preferably more than 25 μm² and 5,000 μm² or less. As the area per elongated grain becomes smaller, breakage and surface roughening are more likely to be suppressed. Thus, 5,000 μm² or less, or 4,800 μm² or less, in particular, 4,500 μm² or less is believed to be preferable. When the area per elongated grain is excessively small, it becomes difficult to obtain the effect of reducing anisotropy in plastic forming. Therefore, more than 25 μm² or 30 μm² or more is believed to be preferable.

In the elongated grains, preferably, the (0001) plane, which is the slip plane in the hexagonal close-packed magnesium alloy, is not parallel, but is inclined with respect to the surface of the magnesium alloy sheet (typically, the rolled surface formed by being brought into contact with a reduction roll). Typically, the (0001) plane is preferably inclined to at least one of the rolling direction and the sheet width direction. In one example, when a pole figure of (0001) planes of elongated grains is taken, and regarding crystal grains with a small angle θ_(TD) in the sheet width direction in the (0001) planes (crystal grains inclined within 5° from the normal direction), the peak of the angle θ_(RD) in the rolling direction in the (0001) planes is checked, the peak is present at a position deviated from the normal direction, and specifically, the position of the peak is at 9° or more. In this example, since there are many crystal grains in which the (0001) planes of elongated grains are inclined to the rolling direction, the effect of reducing anisotropy in plastic forming due to the presence of the inclined crystal grains can also be obtained, and more excellent plastic formability is exhibited. The angle (absolute value) of the position of the peak is preferably as large as possible within a range of 90° or less.

In another example, when a pole figure of (0001) planes of elongated grains is taken, and regarding crystal grains with the angle θ_(RD) in the rolling direction in the (0001) planes in a specific range (crystal grains inclined within 20° from the normal direction), the angle θ_(TD) in the sheet width direction in the (0001) planes is checked, the area fraction of elongated grains with a large angle θ_(TD) in the sheet width direction (elongated grains at 20° or more from the normal direction: TD-inclined elongated grains) is 20% to 70%. TD-inclined elongated grains are crystal grains in which the (0001) plane is particularly largely inclined to the sheet width direction. Since such TD-inclined elongated grains are present in an amount of 20% or more (in terms of area%) relative to all the elongated grains, the effect of reducing anisotropy in plastic forming due to the presence of the TD-inclined elongated grain can also be obtained, and more excellent plastic formability is exhibited. As the area percentage of the TD-inclined elongated grains becomes larger, the effect of reducing anisotropy is more easily obtained. However, strength, impact resistance, and the like are decreased, and the surface texture is degraded. Therefore, 70% or less (in terms of area%) is preferable. More preferably, the area fraction of the TD-inclined elongated grains is 25% to 50%.

Crystal grains other than the elongated grains are each fine and have a structure in which the (0001) plane is oriented parallel to the rolled surface (structure in which the c-axis is oriented perpendicular to the rolled surface). The average crystal grain size of the crystal grains other than the elongated grains is, for example, 1 to 10 μm.

[Magnesium Alloy Structural Member]

The magnesium alloy structural member according to the embodiment is a formed body obtained by subjecting at least part of the magnesium alloy sheet according to the embodiment to plastic forming (in particular, press forming). Examples thereof include a structural member obtained by subjecting the entire magnesium alloy sheet to plastic forming, e.g., a tubular structural member, and a structural member obtained by subjecting only part of the magnesium alloy sheet to plastic forming, e.g., an L-shaped structural member or a structural member having a recessed cross section. A typical example of plastic forming is warm working. The material temperature during plastic forming is 350° C. or lower, preferably 300° C. or lower, and in particular, 150° C. to 280° C., or 150° C. to 220° C. In plastic forming (secondary forming), such as press forming, the period of time in which the material is held at the material temperature described above is relatively short (typically, about several seconds to several minutes, although depending on the forming). Therefore, the magnesium alloy structural member according to the embodiment after plastic forming substantially maintains the composition and structure of the magnesium alloy sheet according to the embodiment, and has excellent strength, rigidity, and impact resistance as in the magnesium alloy sheet according to the embodiment.

The magnesium alloy structural member according to the embodiment may be at least partially subjected to polishing, anti corrosion treatment, such as chemical conversion treatment or anodic oxidation treatment, coating, hairline finish, or decorative treatment, such as diamond cutting or etching. The magnesium alloy structural member may have through-holes, recesses, protrusions, or the like. Furthermore, the magnesium alloy structural member may be joined with a resin formed body.

[Method For Producing Magnesium Alloy Sheet]

The magnesium alloy sheet according to the embodiment having the specific structure described above can be produced, for example, by a production method including the steps described below.

Casting step: a step in which a magnesium alloy is subjected to continuous casting to prepare a cast sheet.

Solution treatment step: a step in which the cast sheet is subjected to solution treatment to produce a solution-treated sheet.

Rolling step: a step in which the solution-treated sheet is subjected to one or more passes of warm rolling.

In particular, the solution treatment is performed such that the average crystal grain size is more than 15 μm and less than 60 μm after the solution treatment. The warm rolling is performed at a preheating temperature of the material of 220° C. to 280° C., at a reduction roll temperature of 200° C. to 300° C., and at a rolling reduction per pass of 30% or less.

(Casting Step)

In the known art, as the cast material, which is a material for casting, an ingot or a material obtained by cutting a thick slab has been used (ingot in Patent Literature 2). In a continuous casting process, since rapid solidification is possible, occurrence of oxides, segregation, and the like can be reduced, and generation of coarse impurities in crystal and precipitated impurities exceeding 10 μm can be suppressed. That is, it is possible to reduce pieces of foreign matter which can be starting points for breakage during rolling. It is also possible to reduce the average crystal grain size to some extent. Therefore, formation of very coarse elongated grains (for example, in which the area per grain is more than 600 μm²) and formation of excessive number of elongated grains are likely to be suppressed. The average crystal grain size of the cast sheet is preferably 15 to 50 μm. The cooling rate (casting velocity) is controlled such that the average crystal grain size of the cast sheet is in the range described above while taking into consideration the composition of the magnesium alloy and the thickness of the cast sheet. In particular, a twin-roll continuous casting process, by which a cast sheet having excellent rigidity and thermal conductivity, little segregation, and an excellent rolling property are likely to be produced, is preferable. Furthermore, in the continuous casting process, a long cast sheet can be easily produced. By using a long cast sheet as the material for rolling, a long rolled sheet can be produced, and productivity of the magnesium alloy sheet according to the embodiment can be improved.

The thickness, width, and length of the cast sheet can be appropriately selected. For example, when the thickness is 10 mm or less, or 7 mm or less, in particular, 5 mm or less, grain refinement by rapid cooling and suppression of segregation can be achieved, and a cast sheet having high strength is likely to be obtained. For example, by producing a long cast sheet with a length of 30 m or more, or 50 m or more, in particular 100 m or more, or a wide cast sheet with a width of 100 mm or more, or 200 mm or more, in particular 250 mm or more, and using it as the material for rolling, a long rolled sheet, or a wide rolled sheet can be produced.

(Solution Treatment Step)

By subjecting the cast sheet to solution treatment, a homogeneous composition can be obtained, improvement in mechanical properties and rolling property due to solid solution of precipitates can be achieved, and the size of crystal grains can be controlled. The solution treatment is performed, for example, under conditions of a heating temperature of 350° C. to 420° C. and a holding time of 1 to 15 hours. Since the solution treatment is performed at a relatively high temperature as described above, as the holding time is increased, crystal grains become more likely to grow. Consequently, elongated grains are likely to be formed, resulting in excessive formation of elongated grains or formation of coarse elongated grains. Therefore, the holding time in the solution treatment is set to be short. Although depending on the composition and thickness of the cast sheet and rolling conditions in the subsequent step, the holding time is more preferably 2 to 12 hours.

The holding time is adjusted within the range described above such that the average crystal grain size of the heat-treated sheet after the solution treatment (solution-treated sheet) is more than 15 μm and less than 60 μm. When the average crystal grain size of the solution-treated sheet is 15 μm or less, crystal grains before rolling are excessively small, and elongated grains are not sufficiently formed after rolling. When strain is introduced by rolling, recrystallization can be caused by the strain. When crystals before recrystallization are excessively small, they do not grow sufficiently even if recrystallization is performed, and it is believed that elongated grains are unlikely to be formed. On the other hand, when the average crystal grain size of the solution-treated sheet is more than 60 μm, crystal grains before rolling are excessively large, resulting in excessive formation of elongated grains and formation of coarse elongated grains. The reason for this is believed to be that, since crystals before rolling are excessively large, strain due to rolling is unlikely to be accumulated, recrystallization due to strain energy does not sufficiently occur, and therefore, coarse crystals remain as they are or coarse crystals are further stretched by rolling. The average crystal grain size of the solution-treated sheet is more preferably 20 to 50 μm.

(Rolling Step)

By subjecting the solution-treated sheet to one or more passes of rolling, it is possible to achieve improvement in mechanical properties due to work hardening, improvement in formability of secondary forming (plastic forming, such as press forming) due to control of the crystalline structure, reduction in the thickness of the sheet, and the like. In particular, at least one pass of rolling is performed by warm rolling. The warm rolling is performed under conditions of a preheating temperature of the material of 220° C. to 280° C., a reduction roll temperature of 200° C. to 300° C., and a rolling reduction per pass of 30% or less. By performing warm rolling under the specific conditions described above on the solution-treated sheet with an average crystal grain size in the specific range, it is possible to obtain the magnesium alloy sheet according to the embodiment having a structure in which elongated grains are present in the specific range in the structure composed of fine crystal grains whose c-axis is oriented perpendicular to the rolled surface. In addition, when warm rolling is performed under the specific conditions described above, the following advantages can be obtained: (1) plastic formability of the material is enhanced, and edge cracking can be reduced; (2) the rolling reduction per pass can be increased (e.g., 10% or more), and productivity can be increased; (3) degradation in the surface texture due to burning or the like can be suppressed; and (4) thermal degradation of reduction rolls can be suppressed.

When heating (preheating) of the material is performed using a heating furnace separately provided, the entire material is likely to be heated uniformly. However, the temperature of the material can be decreased while the material is transported and brought into contact with reduction rolls. Therefore, preferably, the transport distance or transport time may be adjusted, a heat-insulating cover may be provided on the transport path, or the temperature of the atmosphere may be controlled so that the material temperature can be 180° C. or higher immediately before contact with reduction rolls.

Although warm rolling may be performed in all passes of rolling, in the case where rolling with a small rolling reduction is performed, for example, in finish rolling, cold rolling may be performed.

A lubricant is preferably used in the rolling because friction between the material and the reduction rolls is reduced and rolling can be performed satisfactorily.

In the case where rolling is performed with multiple passes, as described in Patent Literature 1, by performing reverse rolling, a long rolled sheet can be produced with high productivity. In the case where reverse rolling is performed, as described in Patent Literature 1, a rolling system may be constructed, which includes a reel for uncoiling the material, a reel for coiling the material, and reduction rolls disposed between the two reels. By using this system and reversing the two reels, reverse rolling with multiple passes can be performed. When each reel is configured to be placed in a heating furnace which preheats the material, a large amount of the material can be preheated at one time, it is possible to shorten the time until the preheated material is introduced between a pair of reduction rolls arranged opposite each other, and a decrease in the temperature of the material can be suppressed. In the case where reverse rolling is performed, a coil spirally wound is used as the material. Furthermore, in the case where reverse rolling is performed, the resulting material is a coil obtained by spirally winding a rolled sheet.

Experimental Example

Magnesium alloy sheets were produced under various conditions, and the cross-sectional structures thereof were examined. Furthermore, the resulting magnesium alloy sheets were subjected to press forming, and plastic formability was evaluated.

In this experiment, a molten metal of a magnesium alloy with a composition corresponding to an AZ91 alloy (Mg-8.7%Al-0.65%Zn, in terms of mass %) was prepared, a cast sheet with a thickness of 4 mm was continuously formed and coiled by a twin-roll casting machine, and thereby, cast coils were produced. In this example, the casting velocity was adjusted so that the average crystal grain size was about 15 to 50 μm. The resulting cast coils were placed in a heating furnace (batch furnace) and subjected to solution treatment to produce solution-treated sheets (solution-treated coils). By varying the conditions for solution treatment, the crystal gain size after the solution treatment was varied. The heating temperature in the solution treatment was selected from the range of 350° C. to 420° C., and the holding time was varied. The holding time of Sample No. 100 was set to be shortest (0.5 hours), and the holding time of Sample No. 200 was set to be longest (100 hours). The holding time of Sample Nos. 1 to 4 was selected from the range to 1 to 15 hours, and as the sample number decreased, the holding time was shortened.

Regarding each of the solution-treated sheets obtained after the solution treatment, the average crystal grain size was measured by the method described below. The results thereof are shown in Table. A specimen for embedding is cut out of each solution-treated sheet so that a cross section parallel to the casting direction and a cross section parallel to the sheet width direction can be observed. The cut-out specimen for embedding is embedded in a resin and subjected to mirror polishing and etching in that order. Then, each cross section is observed with an optical microscope, and the crystal grain size is measured by a line method. A micrograph is taken, at an observation magnification of 100 times, of each of the cross section in the casting direction and the cross section in the sheet width direction. Three line segments corresponding to 1,500 μm are drawn on the micrograph, and the number of crystal grains present on each line segment is counted. The value obtained by dividing the line segment length by the number of crystal grains (line segment length/number of crystal grains) is defined as the crystal grain size on the line segment. The average value of crystal grain sizes of the three line segments in the cross section along the casting direction and crystal grain sizes of the three line segments in the cross section along the sheet width direction is defined as the average crystal grain size.

The resulting solution-treated coils were uncoiled and subjected to warm rolling with multiple passes, and thereby, rolled sheets (rolled coils) were produced. Each of the rolled coils was formed of a rolled sheet with a thickness of 0.8 mm, a width of 250 mm, and a length of 760 m (total rolling reduction: 80%). In this example, a reverse rolling system was used, which included two heating furnaces, each having a built-in reel, and reduction rolls disposed between the two heating furnaces. The material was preheated in the heating furnace for each pass, the material in a heated state was supplied to the reduction rolls, and the travelling direction of the material was changed by reversing the reels. Thus, reverse rolling with multiple passes was performed. For each sample, rolling was performed under conditions of a rolling reduction per pass of 20% to 25%, a preheating temperature of the material of 260° C., and a reduction roll temperature of 250° C.

By appropriately cutting the resulting rolled coils, sheets for structure observation were obtained. Each sheet is cut along a plane parallel to both the thickness direction and the rolling direction to take a longitudinal section. The longitudinal section is observed with a field emission scanning electron microscope (FE-SEM), and the observed image is analyzed and measured using an electron backscatter diffraction (EBSD) technique. Specifically, in a given field of view in the longitudinal section (in this example, one field of view of 3.16×10⁵ μm² (0.316 mm²)), grain particles are identified by crystal grain orientation, and the area of the crystal grain is obtained regarding all the crystal grains in the field of view. Furthermore, elliptical approximation is performed on the outline of each crystal grain, and the length of the major axis (length in the major axis direction): a and the length of the minor axis (length in the minor axis direction: b are obtained. The elliptical approximation is performed by a known method using formulae described below.

The distance d_(ij) between points x_(j) and y_(j) on the ellipse is obtained by Formula 1 below. The maximum value of the distance d_(ij) is equal to the length in the major axis direction: a in the ellipse. The angle γ between the major axis and the horizontal axis is obtained by Formula 2 below. In Formula 2, x_(j) ^(max), y_(j) ^(max), x_(i) ^(max), and y_(i) ^(max) represent two coordinate points, which have the maximum distance. The central coordinates of the ellipse are expressed by Formula 3 and Formula 4. In Formula 3 and Formula 4, x_(k) and y_(k) represent coordinate points of all the data contained in the crystal grains. In order to obtain the length in the minor axis direction: b, x_(k) and y_(k) are converted to the basic coordinate system of the ellipse using Formula 5 and Formula 6. Then, the length of the minor axis: b is obtained using the average formula shown in Formula 7.

$\begin{matrix} {d_{ij} = \sqrt{\left( {x_{i} - x_{j}} \right)^{2} + \left( {y_{i} + y_{j}} \right)^{2}}} & {{Formula}\mspace{14mu} 1} \\ {\gamma = {\tan^{- 1}\frac{y_{j}^{{ma}\; x} - y_{i}^{{ma}\; x}}{x_{j}^{{ma}\; x} - x_{i}^{{ma}\; x}}}} & {{Formula}\mspace{14mu} 2} \\ {\overset{\_}{x} = {\sum\; x_{k}}} & {{Formula}\mspace{14mu} 3} \\ {\overset{\_}{y} = {\sum\; y_{k}}} & {{Formula}\mspace{14mu} 4} \\ {x_{i}^{\prime} = {{\left( {x_{i} - \overset{\_}{x}} \right)\cos \; \gamma} + {\left( {y_{i} - \overset{\_}{y}} \right)\sin \; \gamma}}} & {{Formula}\mspace{14mu} 5} \\ {y_{i}^{\prime} = {{\left( {x_{i} - \overset{\_}{x}} \right)\sin \; \gamma} + {\left( {y_{i} - \overset{\_}{y}} \right)\cos \; \gamma}}} & {{Formula}\mspace{14mu} 6} \\ {b = {\frac{1}{N}{\sum\limits_{i = 0}^{N}\; \sqrt{\frac{{y^{\prime}}_{i}^{2}}{\left( {1 - {{x^{\prime}}_{i}^{2}/a^{2}}} \right)}}}}} & {{Formula}\mspace{14mu} 7} \end{matrix}$

Using the length of the major axis: a and the length of the minor axis: b for each crystal grain, the aspect ratio (length of the major axis/length of the minor axis) is obtained. Elongated grains are selected from the field of view in the longitudinal section on the basis of the aspect ratio. In this example, the elongated grains were selected taking into account the area of crystal grains, in addition to the aspect ratio. Specifically, the average: S_(ave) of areas of all the crystal grains and the standard deviation: σ_(S) of areas of all the crystal grains were obtained, and S_(ave)+3σ_(S) was obtained as the threshold of the area. Then, crystal grains in which the aspect ratio was 3.85 or more and the area was equal to or more than the threshold S_(ave)+3σ_(S) (crystal grains present in a region surrounded by the dashed rectangular frame in FIG. 3(A)) were considered as elongated grains. It is believed that long crystal grains can be more appropriately selected by selecting elongated grains taking into account the area of crystal grains. Note that, without taking into account the area, crystal grains with an aspect ratio of 3.85 or more may be selected as elongated grains. Furthermore, regarding the selected elongated grains, using the area of each elongated grain, the average (average cross-sectional area) was obtained. Furthermore, among the selected elongated grains, the cross-sectional area of the grain having the smallest area (minimum cross-sectional area) and the cross-sectional area of the grain having the largest area (maximum cross-sectional area) were obtained. The results thereof are shown in Table.

FIGS. 1(A), 4(A), and 6(A) are each an inverse pole figure orientation map of elongated grains (FIG. 1(A): Sample No. 2, FIG. 4(A): Sample No. 3, and FIG. 6(A): Sample No. 4). The color key for crystal orientation images is shown below each map. FIGS. 1(B), 4(B), and 6(B) are each a graph showing the relationship between the aspect ratio and the occurrence frequency of crystal grains (FIG. 1(B): Sample No. 2, FIG. 4(B): Sample No. 3, and FIG. 6(B): Sample No. 4).

Regarding the selected elongated grains, a pole figure of (0001) planes of elongated grains was formed, in which, in the sheet for structure observation, the thickness direction was defined as the ND direction (normal direction), the rolling direction was defined as the RD direction, and the sheet width direction was defined as the TD direction. FIGS. 1(C), 4(C), and 6(C) are each a pole figure of (0001) planes of elongated grains (FIG. 1(C): Sample No. 2, FIG. 4(C): Sample No. 3, and FIG. 6(C): Sample No. 4).

In each of the resulting pole figures, crystal grains with the angle θ_(TD) in the sheet width direction (TD direction) in the (0001) planes of the elongated grains within 5° are selected. Specifically, as shown in FIG. 3(B), crystal grains in which the angle θ_(TD) in the TD direction is in the range of −5° to +5° are selected. Then, regarding the angle θ_(RD) in the rolling direction (RD direction) in the (0001) planes of the selected crystal grains, a graph is formed. FIGS. 2(A), 5(A), and 7(A) are each a graph showing the occurrence frequency of the angle θ_(RD) in the RD direction (FIG. 2(A): Sample No. 2, FIG. 5(A): Sample No. 3, and FIG. 7(A): Sample No. 4). Using the resulting graphs, presence or absence of a peak of the angle θ_(RD) in the RD direction was checked, and the angle (inclination angle θ_(P)) from the normal direction at the peak was checked. The results thereof are shown in Table. In this example, since a plurality of peaks are present, the maximum value (large) and the minimum value (small) of the inclination angle θ_(P) (absolute value) are shown in Table.

Furthermore, in each of the resulting pole figures, crystal grains with the angle θ_(RD) in the rolling direction (RD direction) in the (0001) planes of the elongated grains within 20° are selected. Specifically, as shown in FIG. 3(B), crystal grains in which the angle θ_(RD) in the RD direction is in the range of −20° to +20° are selected. Then, regarding the angle θ_(TD) in the TD direction in the (0001) planes of the selected crystal grains, a graph is formed. FIGS. 2(B), 5(B), and 7(B) are each a graph showing the occurrence frequency of the angle θ_(RD) in the TD direction (FIG. 2(B): Sample No. 2, FIG. 5(B): Sample No. 3, and FIG. 7(B): Sample No. 4). Using the resulting graphs, the total area fraction ΣS₂₀ of crystal grains with the angle θ_(TD) in the TD direction from the normal direction being −20° or less and crystal grains with the angle θ_(TD) in the TD direction from the normal direction being +20° or more was checked. The total area was obtained by integrating the hatched regions in each of FIGS. 2(B), 5(B), and 7(B). The results thereof are shown in Table.

The areas of crystal grains, elliptical approximation, calculation of the length of the major axis, the length of the minor axis, and the aspect ratio, selection of elongated grains, formation of pole figures, the inclination angle θ_(P) at the peak of the angle θ_(RD) in the RD direction, and the total area fraction ΣS_(N) of crystal grains can be easily and automatically obtained using commercially available mathematical software accompanying a commercially available SEM-EBSD system. In this example, SUPRA35VP manufactured by Carl Zeiss was used as the SEM, and OIM Analysis 5.31 manufactured by EDAX-TSL was used as the software of EBSD.

The resulting rolled coils were subjected to straightening and then surface polishing. The polished sheets were subjected to press forming, and press formability was evaluated as plastic formability. The straightening was performed, using a known roll leveler (refer to Patent Literature 1), under a warm condition (roll temperature: 250° C.). Regarding the polishing, wet polishing was performed using an abrasive belt (polishing amount: about 30 μm in total for both sides).

Press formability was evaluated in terms of (1) limiting drawing ratio,(2) breakage caused by pressing, and (3) surface roughness in press-formed portion. Press forming conditions are shown below.

(1) Limiting drawing ratio: Using a cylindrical punch with a diameter of 50 mm and a shoulder R of 2 mm, a cupping test is conducted under a warm condition (250° C.). As the materials for drawing, by cutting the polished sheet described above into a circular shape, circular sheets with various diameters D (mm) were prepared. Then, the limiting drawing ratio (LRD) was checked. The limiting drawing ratio is defined as the ratio, material diameter Dmax/punch diameter d (50 mm in this example).

(2) Breakage caused by pressing: Using a prismatic punch with a punch R of 0 mm, a right-angle bending test is conducted under a warm condition (250° C.). As the material for bending, a rectangular sheet was prepared by cutting the polished sheet to a predetermined length (length 200 mm). After the sheet was bent at a right angle, presence or absence of cracks on the outer peripheral surface of the bent portion was checked by visual observation. The material with no breakage is evaluated to be good (◯).

(3) Surface roughness in press-formed portion: In the material subjected to the right-angle bending test described in (2), surface roughness of the outer peripheral surface of the bent portion was measured. The surface roughness, in terms of arithmetic mean roughness Ra, was measured, using a commercially available surface roughness tester, in accordance with JIS B 0601(2001)/ISO 4287(1997).

TABLE Solution- Rolled sheet Rolled sheet treated Rolled Rolled TD: −20° elongated grains Press Press- sheet sheet area sheet peak or less Average Minimum Maximum formability formed average fraction of inclination TD: +20° cross- cross- cross- evaluation Breakage portion Compre- crystal elongated angle or more sectional sectional sectional limiting caused surface hensive Sample grain size grains θ _(P)(° ) area fraction area area area drawing by roughness evalu- No. (μm) (%) Small Large Σ S₂₀ (%) (μm²) (μm²) (μm²) ratio pressing Ra (μm) ation 100 15 1.5 10.5 14.7 16.8 59 25 245 2 ◯ 0.5 Δ 1 20 3.1 10.1 15.8 20.5 101 30 504 2.3 ◯ 0.7 ◯ 2 30 8.2 9.9 15.9 26.7 167 55 724 2.5 ◯ 0.9 ⊚ 3 40 12.2 12.9 13.9 48.6 280 75 1370 2.3 ◯ 1.6 ◯ 4 50 19.7 10.9 14.9 64.0 575 139 4140 2.3 ◯ 1.9 ◯ 200 60 25.6 11.0 16.1 71.0 740 185 5430 2 Cracks 2.5 X occurred Obvious surface roughening

As shown in Table, in Sample Nos. 1 to 4 having a cross section (cross section parallel to both the thickness direction and the rolling direction in this example) in which a plurality of elongated grains with an aspect ratio of 3.85 or more are present and the area fraction of the elongated grains is 3% to 20%, excellent formability of plastic forming, such as press forming, is exhibited. In this example, in Sample Nos. 1 to 4, the limiting drawing ratio is large at more than 2, high deformation is possible, cracks are unlikely to occur even when subjected to right-angle bending, the bent portion is smooth, and the surface texture is excellent.

Such magnesium alloy sheets having excellent plastic formability have a structure in which fine crystal grains with a low aspect ratio and long crystal grains (elongated grains) are mixed as shown in FIGS. 1(A), 1(B), 4(A), 4(B), 6(A), 6(B), and the like. From this, it is believed that in Sample Nos. 1 to 4, since irregular-shaped crystal grains are included in a specific range, anisotropy in plastic forming is reduced and plastic formability is enhanced compared with the case where a sheet is composed of fine crystal grains with a uniform shape. Furthermore, in this example, many elongated grains have the (0001) plane that is inclined to both the rolling direction and the sheet width direction, that is, are crystal grains whose c-axis is inclined to the rolled surface. Specifically, the inclination angle θ_(P) is 9° or more (in this example, both the maximum value and the minimum value of the inclination angle θ_(P) are 9° or more), and the total area fraction ΣS₂₀ satisfies a range of 20% to 70%. It is believed that, in Sample Nos. 1 to 4, since such crystal grains whose c-axis is inclined to the rolled surface are included in a specific range, anisotropy in plastic forming can be further reduced, and plastic formability can be further enhanced. Furthermore, it is believed that, since the average cross-sectional area of the elongated grains is 600 μm² or less, the elongated grains are unlikely to act as starting points for breakage, and plastic formability can be enhanced. Furthermore, it is believed that, since the cross-sectional area of each of the elongated grains is more than 25 μm² and 5,000 μm² or less, the elongated grains are unlikely to act as starting points for breakage, and plastic formability can be enhanced.

Test pieces were made from the resulting polished sheets, and using a commercially available tensile tester, tensile strength (room temperature) and 0.2% proof stress (room temperature) were measured. As a result, Sample Nos. 1 to 4 each had a tensile strength of 270 MPa or more and a 0.2% proof stress of 220 MPa or more, indicating high strength. The reason for such results is believed to be that, since the content of elongated grains whose c-axis is oriented non-parallel to the rolled surface is within the specific range, and substantially all of crystal grains other than the elongated grains are fine with the c-axis being oriented perpendicular to the rolled surface, high strength can be maintained. Furthermore, by using such a magnesium alloy sheet having high strength as the material, it is anticipated that the magnesium alloy structural member subjected to press forming will have high strength and excellent impact resistance and will be unlikely to be dented.

The magnesium alloy sheet having excellent plastic formability as described above can be produced by subjecting a continuously cast material to solution treatment, setting the crystal grain size after the solution treatment within a specific range, and controlling the preheating temperature of the material and the reduction roll temperature during rolling to specific ranges.

It is to be understood that the present invention is not limited to the embodiments described above, but the embodiments can be appropriately changed within a range not departing from the gist of the present invention. For example, the composition of the magnesium alloy, the thickness, width, and length of the sheet, production conditions (solution treatment temperature/holding time, the rolling reduction per pass, the material temperature and reduction roll temperature during rolling, and the total rolling reduction), and the like can be appropriately changed.

INDUSTRIAL APPLICABILITY

The magnesium alloy sheet according to the present invention can be suitably used as the material for magnesium alloy structural members which have been subjected to various types of plastic forming, such as press forming, e.g., bending, drawing, and shearing, forging, and upsetting. The magnesium alloy structural member according to the present invention can be suitably used for structural members constituting various electric/electronic devices (more specifically, housings and reinforcing members of mobile and small electric/electronic devices, and the like), structural members constituting transportation apparatuses, such as automobiles and aircraft, exterior structural members, such as various housings and covers, framework parts, bags, and the like. 

1. A magnesium alloy sheet obtained by subjecting a magnesium alloy to rolling, and having a cross section parallel to the thickness direction of the magnesium alloy sheet in which the area fraction of elongated grains in the cross section is 3% to 20%, where the elongated grains are defined as crystal grains having an aspect ratio of 3.85 or more in the cross section when the length of the major axis and the length of the minor axis of each of crystal grains in the cross section are determined, and the aspect ratio is defined as the ratio of the length of the major axis to the length of the minor axis.
 2. The magnesium alloy sheet according to claim 1, wherein, when a pole figure of (0001) planes of the elongated grains is taken, crystal grains with the angle θ_(TD) in the sheet width direction in the (0001) planes of the elongated grains within 5° are selected, and the angle θ_(RD) in the rolling direction in the (0001) planes of the selected crystal grains is checked, the peak of the angle θ_(RD) in the rolling direction is present at 9° or more from the normal direction.
 3. The magnesium alloy sheet according to claim 1, wherein, when a pole figure of (0001) planes of the elongated grains is taken, crystal grains with the angle θ_(RD) in the rolling direction in the (0001) planes of the elongated grains within 20° are selected, and the angle θ_(TO) in the sheet width direction in the (0001) planes of the selected crystal grains is checked, the total area fraction of crystal grains with the angle θ_(TD) in the sheet width direction from the normal direction being −20° or less and crystal grains with the angle θ_(TD) in the sheet width direction from the normal direction being +20° or more is 20% to 70% relative to all the elongated grains.
 4. The magnesium alloy sheet according to claim 1, wherein the average cross-sectional area of the elongated grains is 600 μm² or less.
 5. The magnesium alloy sheet according to claim 1, wherein the magnesium alloy contains 8.3% to 9.5% by mass of Al.
 6. The magnesium alloy sheet according to claim 1, wherein the cross-sectional area of each of the elongated grains is more than 25 μm² and 5,000 μm² or less.
 7. A magnesium alloy structural member obtained by subjecting at least part of the magnesium alloy sheet according to claim 1 to press forming. 